We had some reports that when trying to sign in to Skype for Business Online users were experiencing delays during the sign in process.
This behaviour was related to environments were the Lync/SfB client is configured to use a Proxy Server to connect to the Sype for Business Online servers.
After researching the delay was occurring because the client was trying establish a direct connection and only after that connection timed out it would try to connect using the configured Proxy Server:
As a workaround we could configure the Firewalls to send RESET Packets and the Lync/SfB client won’t wait for the connection attempt to timeout.
Since the following client updates we can use a registry key to force the client to use proxy for all connections:
December 6, 2016, update for Skype for Business 2015 (Lync 2013) (KB3127976)
Office 365 ProPlus/Office Professional Plus 2016 Click-to-Run
Version 1611 (Build 7571.2072) – December 6, 2016
To configure Lync/SfB client to use a proxy server for SIP Traffic without attempting a direct connection we need to add the following registry key:
reg add HKCU\Software\Microsoft\UCCPlatform\Lync /v EnableDetectProxyForAllConnections /t REG_DWORD /d 1 /f
Note: This registry key is only available on a User Level, we cannot add it under HKEY_Local_Machine.